It’s difficult enough designing robots that can maneuver on dry land. Imagine designing medical bots able to move around in the wet, goopy environment inside the human body? A metal-based robot would face corrosion and stiff hard edges aren’t exactly body-friendly.

Some scientists think squishy robots made from biocompatible, water-based gels are the answer. And a team from North Carolina State University has found a way to get these otherwise unmanageable, floppy bots to stiffen up and grasp objects. The technique could lead to soft robots with parts that move like artificial muscles.

“In the nearer term, the technique may have applications for drug delivery or tissue scaffolding and directing cell growth in three dimensions, for example,” professor of chemical and biomolecular engineering, Orlin Velev, said in a press release.

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The researchers cut the hydrogel into different shapes to test their idea. First, they made a V-shaped piece of hydrogel and injected copper ions into one side. The V’s “arms” flexed, allowing it to grasp. When they ran the current through the other side of the V, it released.

The flexing happened because the polymer molecules in the gel have positively and negatively charged regions, and the copper ions linked to the latter. Since a positive copper ion was linked to a polymer molecule, another polymer molecule could attach itself to the other side of the copper ion. The copper linked the polymer molecules together, and the hydrogel became stiffer and tougher in one region.

The copper didn’t just make hydrogels stiffer, though. The copper-linked polymer molecules made long strands, and those strands were attracted to one another by their copper links. As a result the hydrogel bent, and putting more current in the electrode, sending more copper ions into the gel, made it bend more.

The researchers also found that it wasn’t even necessary to run current through the gel: movement could also be induced with chemicals. In a second experiment, the team made an X-shaped piece of hydrogel with a pattern of stiffened gel on one side. When they immersed it in alcohol, the hydrogel without the pattern shrank while the stiffened hydrogel retained its shape. That caused the “arms” of the X to bend, allowing it to grab a small object. The scientists then put the X in water. The non-stiffened hydrogel expanded, and the small object was released.

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Both processes are similar to the electrical activity in muscles, in which a current or chemical induces flexing and relaxing.

Copper isn’t the only ion they could do this with. In a press release they said Calcium would work as well.

The experiments are detailed in the Aug. 2 online issue of Nature Communications.

Image: North Carolina State University